WO2020016709A1

WO2020016709A1 - Heterogeneous catalysts and process based on supported/grafted titanium hydrides for catalytic ammonia formation from nitrogen and hydrogen

Info

Publication number: WO2020016709A1
Application number: PCT/IB2019/055892
Authority: WO
Inventors: Balamurugan VIDJAYACOUMAR; Khalid Albahily; Jean-Marie Basset; Janet Chakkamadathil MOHANDAS; Manoja SAMANTARAY
Original assignee: Sabic Global Technologies B.V.
Priority date: 2018-07-19
Filing date: 2019-07-10
Publication date: 2020-01-23

Abstract

Embodiments of the invention are directed to metal hydride catalysts that are operable under ambient conditions and provide for a more energy efficient production of ammonia from a nitrogen and hydrogen containing feed source.

Description

HETEROGENEOUS CATALYSTS AND PROCESS BASED ON SUPPORTED/GRAFTED TITANIUM HYDRIDES FOR CATALYTIC AMMONIA

FORMATION FROM NITROGEN AND HYDROGEN

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/700,512 filed July 19, 2018, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns catalysts for the production of ammonia. In particular the catalysts are operable under ambient conditions and provide for a more energy efficient production of ammonia from nitrogen.

B. Description of the Related Art

[0003] Synthesis of ammonia conventionally is performed by passing a process stream of synthesis gas, containing nitrogen and hydrogen in essentially stoichiometric amounts, to the formation of ammonia through a catalyst arranged in one or more beds in a reactor. Thermodynamically, the reaction of hydrogen and nitrogen to ammonia requires the reaction to be performed at elevated pressure (Lechatelier principle), usually above 100 atmospheres. But the reaction is exothermic and requires in theory lower temperatures. Bur for kinetic reason the temperature is elevated between 300 °C and 600 °C. Under these process conditions, the equilibrium concentration of ammonia in a stoichiometric synthesis gas composition is below 20% by volume in the synthesis gas stream from the reactor. The stream has to be recycled to the reactor, together with fresh synthesis gas, in order to provide a reasonably sufficient ammonia product yield. Prior to recycling, the stream is cooled to separate ammonia from unreacted hydrogen, nitrogen, and inert diluents present in the synthesis gas. A fraction of the recycle gas is purged to vent inert gases. The stripped gas is then passed to a compression stage by which it is recycled to the reactor. The expense of compressing and recycling synthesis gas are important factors in the economy of ammonia production in general, and in particular when production capacities of existing ammonia synthesis loops have to be increased. The existing processes for ammonia production are energy intensive.

[0004] Strategies underlying these processes for ammonia production include multi- promoted Fe based supported systems. These systems are used because the catalyst component is abundant, cheap, and convenient to handle under the reaction conditions. Hence, the tremendous amount of energy that was consumed for this equilibrium conversion was viewed as acceptable. In view of its known advantages, iron (Fe) based catalysts still require a minimum of 400 °C and 200 atmospheres to have an appreciable extent of activity.

[0005] Homogeneous catalysis to produce ammonia has been investigated. By way of example, Schrock et al. Science 2003, 307, pp. 76-78 describes a Mo complex that can catalytically transform dinitrogen to ammonia with a turn-over-number (TON) of 6 under ambient conditions. This process suffers from practical utility due to the complexity of the process.

[0006] Cleavage of the dinitrogen bond to produce oraganometallic nitrogen complexes has been described. By way of example, Avenier et al. {Science 2007, 377, pp. 1056-1060) describes cleavage of N2 at 250 °C and atmospheric pressure by H2 on an isolated silica surface- supported organometallic hydride centers, leading to an organometallic amido imido complex. In yet another example, of dinitrogen cleavages, Jia et al. (Inorg. Chem. 2015, Vol. 54, pp. 11648-11659) describes the organometallic amino reaction products of organometallic hydride complexes with hydrazines. Neither of these references describe the production of ammonia from cleavage of N2.

[0007] Accordingly, the need exists for more cost effective and energy efficient ammonia production processes.

SUMMARY OF THE INVENTION

[0008] New catalysts and processes have been developed that provide a solution to the some of the economic and energy efficiency problems associated with ammonia production. In particular, catalyst and processes for the ammonia formation reaction have been developed that work at lower temperatures as well as lower pressures. The catalysts described herein are operable under ambient conditions and provide for a more energy efficient production of ammonia from nitrogen, operating at lower temperatures and pressures than some of the existing catalyst used in the production of ammonia. The catalyst described herein are developed using a surface organometallic chemistry (SOMC) strategy and can have a turn over number as high as 30 under ambient reaction conditions. The catalysts described herein can be utilized in a crystalline or amorphous form. The catalyst can be supported on an inorganic oxide, a metal, a carbon, or combinations of these materials. In certain aspects of the present invention, the support is of alumina, silica, titania, zirconia, MCM-41 or combinations thereof. [0009] The catalytic elements can be dispersed as mechanically mixed powders, or can be chemically dispersed, impregnated or deposited. When mixed powders are used in the present invention, the powder particle size is controlled to provide a powder that has particles that are small enough to provide suitable surface area and reactivity, but not so fine as to produce significant surface oxidation.

[0010] Aspects of the invention include a metal hydride catalyst comprising a support having [(ºSi-0-)xMH_y][SiH_z] on the surface of the support, where M is a transition metal selected from titanium (Ti), zirconium (Zr), or hafnium (Hf), x can be 1 to 3 (e.g., 1, 2, 3), and y can be 1 to 3 (e.g, 1, 2, 3), and z can be 1 to 3 (e.g, 1, 2, 3). In certain aspects, M is Ti, Zr, or Hf. In particular aspects M is Ti. The support can include silica. The support can further include other inorganic oxides, a metal, a carbon, or a combination thereof. In certain aspects, the support further includes alumina, titania, zirconia, chromia, or combinations thereof. The support can have a specific surface area (BET) in a range of 0.1 to 1200 m²/g. The support can be a non-porous silicate, mesoporous silicate, a microporous silicate, or zeolite support.

[0011] Certain aspects are directed to methods of producing a transition metal hydride catalyst comprising: (a) grafting a transition metal precursor (titanium (Ti) precursor, zirconium (Zr) precursor, hafnium (Hf) precursor, precursor, or combinations thereof) to a support at a temperature between -80 °C to 300 °C, and (b) activating the grafted precursor by contacting the grafted precursor with H₂ at a temperature of 25 °C to 250 °C preferably, from 100 °C to 200 °C. In certain aspects, the molar ratio of a transition metal hydride precursor to anchoring surface groups of the support is about 0.1 : 1, 0.5: 1, 1 : 1, 2: 1, or 10: 1, including all values and ranges there between. The activating temperature can be about 150 °C. In certain aspects, the activating temperature is in the range of 125 to 175 °C. The metal precursor can include a transition metal selected from Ti, Zr, Hf, or combinations thereof with alkyl, or alkylidene, or alkylidyne or allyl or benzyl ligands. In certain aspects, the metal precursor has a formula of M(R¹)₄, where M is Ti, Zr, Hf, or combinations thereof, and R¹ is a saturated or unsaturated C3 to C10 linear or branched alkyl residues. In a particular aspect, the metal precursor is M(CH₂CMe3)₄ where M is Ti, Zr, Hf, or combinations thereof. In a further aspect, M is Ti

[0012] The methods of producing a metal hydride catalyst can further include preparing the M(R^x)₄ precursor by reacting M(OR²)₄ with Li(R¹) or Zn(R²)₂, wherein M is Ti, Zr, Hf, or Rf, R² is a C₂ to C₄ alkyl, and R¹ is saturated or unsaturated C3 to C10 linear or branched alkyl. The support can be an inorganic oxide, a metal, a carbon, or a combination thereof, which has a specific surface area (BET) in a range of 0.1 to 1200 m²/g. In certain aspects a silica support is a non-porous silicate, a mesoporous silicate, a microporous silicate, or a zeolite support.

[0013] Certain aspects are directed to a transition metal hydride catalyst produced by the methods described herein. In certain aspects transition metal hydride catalyst, under infrared spectroscopy, exhibits at least one or more absorption bands at approximately 1642 cm ¹ and 1686 cm ¹; under proton nuclear magnetic resonance (solid ¹H-NMR) at 400 MHz, exhibits chemical shift values at approximatle 8.5 ppm and 14 ppm corresponding to the hydrides.

[0014] Certain aspects are directed to methods of producing ammonia comprising contacting nitrogen (N2) and hydrogen (H2) with a transition metal hydride catalyst described herein at a temperature of 20 to 350 °C and a pressure of 1 to 15 bar producing ammonia (NH3). The transition metal hydride catalyst can include [(ºSi-0-)xMH_y][SiH_z] coupled to a support, where M is Ti, Zr, Hf, or combinations thereof, x is 1 to 3, y is 1 to 3, and z is 1 to 3. The transition metal hydride catalyst can include [(ºSi-0-)xMH_y][SiH_z], where x is 1, y is 2, and z is 2. In certain aspects, M is Ti.

[0015] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention.

[0016] The following includes definitions of various terms and phrases used throughout this specification.

[0017] The use of the word“a” or“an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean“one,” but it is also consistent with the meaning of“one or more,”“at least one,” and“one or more than one.”

[0018] The terms“about” or“approximately” are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%. [0019] The use of the term“or” in the claims is used to mean“and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and“and/or.”

[0001] The terms “wt.%”, “vol.%”, or“mol.%” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component.

[0002] The term“substantially” and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0020] The terms“inhibiting” or“reducing” or“preventing” or“avoiding” or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

[0021] As used in this specification and claim(s), the words“comprising” (and any form of comprising, such as“comprise” and“comprises”),“having” (and any form of having, such as “have” and“has”),“including” (and any form of including, such as“includes” and“include”) or“containing” (and any form of containing, such as“contains” and“contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0022] The catalysts of the present invention can“comprise,”“consist essentially of,” or “consist of’ particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phrase“consisting essentially of,” in one non limiting aspect, a basic and novel characteristic of the catalysts of the present invention are their abilities to catalyze production of ammonia from nitrogen and hydrogen.

[0023] In the context of the present invention, at least twenty embodiments are now described. Embodiment 1 is a transition metal hydride catalyst containing a support having [(ºSi-0-)xMH_y][SiHz] on the surface of the support, where M is a Colum 4 transition metal, x is 1 to 3, y is 1 to 3, and z is 1 to 3. Embodiment 2 is the catalyst of embodiment 1, wherein M is Ti, Zr or combinations thereof, preferably Ti. Embodiment 3 is the catalyst of embodiment 1, wherein in the support is a non-porous silicate, mesoporous silicate, a microporous silicate, or zeolite support. Embodiment 4 is the catalyst of embodiment 1, wherein the support has a specific surface area (BET) in a range of 0.1 to 1200 m²/g. Embodiment 5 is the catalyst of any of embodiments 1 to 4, wherein the support further contains of alumina, titania, zirconia, chromia or combinations thereof.

[0024] Embodiment 6 is a method of producing a transition metal hydride catalyst of embodiments 1 to 5. The method includes the steps of (a) grafting a Column 4 transition metal precursor to a silica support at a temperature between -80 °C to 300 °C; and (b) activating the grafted precursor by contacting the grafted precursor with Eh at a temperature of 25 °C to 250 °C preferably, from 100 °C to 200 °C. Embodiment 7 is the method of embodiment 6, wherein the Column 4 transition metal hydride precursor to silica molar ratio is about 1 : 1 to the amount of anchoring surface groups on the silica. Embodiment 8 is the method of embodiment 6, wherein the activating temperature is about 150 °C. Embodiment 9 is the method of embodiment 6, wherein Column 4 transition metal precursor contains a Column 4 transition metal with alkyl, alkylidene, or alkylidyne or allyl or benzyl ligands. Embodiment 10 is the method of embodiment 6, wherein the Column 4 transition metal precursor has a formula of M(R¹)₄, where M is a Column 4 transition metal and R¹ is a saturated or unsaturated C3 to C10 linear or branched alkyl or alkyl residues. Embodiment 11 is the method of embodiment 10, wherein the Column 4 transition metal precursor is M(CH₂CMe3)4 where M is the Column 4 transition metal. Embodiment 12 is the method of embodiments 6 to 11, where M is Ti, Zr or combinations thereof. Embodiment 13 is the method of embodiment 6 to 12, wherein the Column 4 transition metal is titanium (Ti). Embodiment 14 is the method of embodiment 6, further including the steps of preparing the M^R¹^ precursor by reacting M(OR²)₄ with Li-(R¹) or Zn (R²)₂, where M is the column 4 transition metal, R² is a C₂ to C₄ alkyl, and R¹ is saturated or unsaturated C3 to C10 linear or branched alkyl. Embodiment 15 is the method of embodiments 6 to 14, wherein the support has a specific surface area (BET) in a range of 0.1 to 1200 m²/g. Embodiment 16 is the method of embodiments 6 to 14, wherein the support is non-porous silicate, mesoporous silicate, microporous silicate, or zeolite support.

[0025] Embodiment 17 is a transition metal hydride catalyst produced by the method of embodiment 6, under infrared spectroscopy, exhibits at least one or more absorption bands at 1642 cm ¹ and 1686 cm ¹; under proton nuclear magnetic resonance (solid 'H-NMR) at 400 MHz, exhibits chemical shift values at 8.5 ppm and 14 ppm corresponding to the hydrides.

[0026] Embodiment 18 is a method of producing ammonia including the steps of contacting nitrogen (N₂) and hydrogen (H₂) with a transition metal hydride catalyst at a temperature of 20 to 350 °C and a pressure of 0.1 to 1.5 MPa producing ammonia (NH3), wherein the transition metal hydride catalyst contains [(ºSi-0-)xMH_y][SiH_z] on the surface of the support, where M is a Column 4 transition metal; x is 1 to 3; and y is 1 to 3, and z is 1 to 3. Embodiment 19 is the method of embodiment 18, wherein the Column 4 transition metal hydride catalyst contains [(ºSi-0-)xMHy][SiH_z], where x is 1, y is 2, and z is 2. Embodiment 20 is the method of embodiments 18 to 19, wherein M is Ti.

[0027] Other obj ects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0029] FIG. 1 is an illustration of an example of a reaction strategy (Scheme I) of the present invention.

[0030] FIGS. 2A and 2B show proton (¾) and carbon 13 (¹³C) NMR spectra of [Ti(CH₂CMe3)4] (2A) ¾ and (2B) ¹³C.

[0031] FIGS. 3A and 3B show (3A) ¾ and (3B) ¹³C NMR spectra of

[(ºSiO-)x[Ti(CH₂CMe3)4]. CH₂CMe₃)4 is abbreviated as Np_3.

[0032] FIG. 4 shows 'H NMR spectra of [(ºSiO-)xTiH_y][SiH_z] prepared from

[(ºSiO-)xTiNp₃] at 100 °C.

[0033] FIG. 5 shows ¾ NMR spectra of [(ºSi-0-)_xTiH_y] prepared from [(ºSiO-)_xTiNp₃] at 150 °C.

[0034] FIG. 6 shows IR spectra of [(ºSi-0-)TiNp₃][SiH_z].

[0035] FIG. 7 shows IR spectra of [(ºSi-0-)_xTiH_y][SiH_z] prepared at 100 °C.

[0036] FIG. 8 shows IR spectra of [(ºSi-0-)_xTiH_y][SiH_z] prepared at 150 °C. [0037] FIGS. 9A and 9B show (9 A) representation of an active catalyst surface and (9B) IR spectra of surface reaction monitoring of [(ºSi-0-)xTiH_y][SiH_z] prepared by 100 °C hydrogenolysis on reaction with N2 and H2 under various heat treatment temperatures.

[0038] FIGS. 10A and 10B show (10A) representation of an active catalyst surface and (10B) IR spectra of surface reaction monitoring of [(ºSi-0-)xTiH_y][SiH_z] prepared by 150 °C hydrogenolysis on reaction with N2 and H2 under various heat treatment temperatures.

[0039] FIGS. 11A and 11B show (11 A) representation of an active catalyst surface and (11B) catalytic activity plot of catalyst prepared at 100 °C.

[0040] FIGs. 12A and 12B show (12A) Representation of an active catalyst surface and (12B) catalytic activity plot of catalyst prepared at 150 °C.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The design and production of catalysts capable of activating and fixing nitrogen at low temperatures as well as lower pressures is the subject of the current application. The observation of splitting N2 on TaH_x grafted MCM-41 surface by Avenier el al ( Science 2007, 317: 1056-60) opened up many possibilities for making use of the surface organometallic chemistry for this classical chemistry. Later, in the inventors’ studies related to generating TaHx species on S1O2-7OO surface from a much more reactive TaMes grafted on fumed silica (e.g, Aerosil® SiO2-700 (Evonik Resource Efficiency, GmbH, Germany), the inventors observed that N2 could not only be split on the surface by the reaction with N2 and H2, but also could be fixed to ammonia at room temperature and atmospheric pressure. However, the amount of ammonia detected was sub -stoichiometric, thus limiting the scope of developing a catalytic process using TaH_x species under the reaction conditions used.

[0042] It was surprisingly found that hydrogenating silica supported Column 4 metals used catalytic conversion of nitrogen to ammonia under mild reaction conditions occurred. By varying the hydrogenation temperature from 100 to 150 °C, the extent of hydrides as well as the extent of ammonia formed was increased substantially. A non-limiting example of these results is described in detail in the Examples section.

A. Catalyst and Catalyst Production

[0043] The supported metal hydride catalyst of the present invention can have the formula [(ºSi-0-)xMH_y][SiH_z], where M is a Column 4 of the Periodic Table transition metal (i.e., titanium (Ti), zirconium (Zr), hafnium (Hf) and combinations thereof), x is 1 to 3, y is 1 to 3, and z is 1 to 3. The support can be silica and silica hydride. The support can also include a metal, carbon, alumina, titania, zirconia, chromia, or combinations thereof. The support can have a specific surface area (BET) in a range of 0.1, 1.0, 10, 100, 400, 500 to 600, 700, 800, 900, 1000, 1100, 1200 m²/g. The support can be a non-porous silicate, mesoporous silicate, a microporous silicate, or zeolite support.

[0044] The metal hydride catalyst can be produced by (a) grafting a transition metal precursor (titanium (Ti), zirconium (Zr), hafnium (Hf), or combinations thereof) to a support at a temperature between -80, -50, 0, 10, 50, 100 °C to 120, 150, 200, 250, 300 °C, and (b) activating the grafted precursor by contacting the grafted precursor with Eh at a temperature of 25, 50, 75, 100, 125 °C to 150, 175, 200, 225, 250 °C preferably, from 100 °C to 200 °C. The molar ratio of the transition metal hydride precursor to anchoring surface groups of the support can be about 0.1 : 1, 0.5: 1, 1 : 1, 2: 1, or 10: 1, including all values and ranges there between. The activating temperature can be at least, equal to, or between any two of 100 °C, 125 °C, 150 °C, 175 °C. In certain aspects, the activating temperature is in the range of 125 °C to 175 °C, particularly about 150 °C. The metal precursor can be a transition metal selected from Ti, Zr, Hf, with alkyl, or alkylidene, or alkylidyne or allyl or benzyl ligands. In certain aspects, the metal precursor has a formula of M(R¹)₄, where M is a metal selected from Ti, Zr, Hf, or combinations thereof, R¹ is a saturated or unsaturated C3 to C10 linear or branched alkyl residues. In a particular aspect, the metal precursor is M(CH₂CMe3)4 where M is a metal selected from Ti, Zr, Hf, or combinations thereof. In a further aspect M is Ti.

[0045] The methods of producing a metal hydride catalyst can further include preparing the M(R^x)₄ precursor by reacting a metal alkyoxy material M(OR²)₄ where M is a metal selected from Ti, Zr, Hf, or combinations thereof, R² is a C2 to C₄ alkyl, with alkyl anion. The alkyl aninon can be Li(R¹) or Zn(R²)₂, where R¹ is saturated or unsaturated C3 to C1₀ linear or branched alkyl and R² is a C₂ to C₄ alkyl group. The support can be include silica. The silica support can be a non-porous silicate, a fibrous silica, a mesoporous silicate, a microporous silicate, or a zeolite support. The support can have a specific surface area (BET) in a range of 0.1, 1.0, 10, 100, 200, 300, 400, 500, 600, 700, 800 900, 100, 1100, to 1200 m²/g, including all values and ranges there between. Infrared spectroscopy of one of the metal hydride catalyst described herein exhibits at least one or more absorption bands at around 1642 cm ¹ and 1686 cm ¹. In a further aspect, proton nuclear magnetic resonance (solid ¹H-NMR) at 400 MHz exhibits chemical shift values at around 8.5 ppm and 14 ppm corresponding to the hydrides. B. Production of Ammonia

[0046] Certain asepcts of the invention are directed to production of ammonia by catalytic conversion of a nitrogen/hydrogen gas mixture in the presence of a metal hydrid catalyst as described above. The process can include passing a feed stream containing nitrogen and hydrogen through a reactor contacting the feed stream with a metal hydrid catalyst under ammonia-forming conditions to generate a product stream having a greater ammonia- concentration than the feed stream. In certain aspects the reactor can be at a temperature of 20 to 400 °C and a pressure of 1 to 15 atm. In certain instances the feed stream can be preheated or equilibrated prior to introduction into the reactor. In a further aspect the reactor is a fixed bed or tubular reactor.

[0047] The reaction can include contacting nitrogen (N2) and hydrogen (H2) at a molar ratio of about 5: 1, 4: 1, 3: 1, 2:1, 1 :1 with a metal hydride catalyst described herein at a temperature of 20 to 350 °C and a pressure of 1 to 15 atm (0.10 MPa to 1.5 MPa) to produce ammonia (NH3). When the supported transition metal hydride catalyst is exposed to hydrogen a portion of the silica form silica hydride in situ , and thus, an actived the transition metal hydride catalyst of [(ºSi-0-)xMHy][SiHz], where M is Ti, Zr, Hf, or or combinations thereof, x is 1 to 3, y is 1 to 3, and z is 1 to 3 can be formed. The transition metal hydride catalyst can be [(ºSi- 0-)xMHy][ ºSiHz], where x is 1, y is 2, and z is 2. In certain aspects M is Ti.

[0048] In one aspect, a system can include a separator for separating the ammonia from by- products, the separator having an inlet connected to the reactor for receiving the ammonia and by-products, a first outlet connected to an ammonia storage tank for collecting the ammonia, and a second outlet connected to a recycle loop for re-circulating by-product back to the inlet of the reactor.

Examples

[0049] The following examples as well as the figures are included to demonstrate certain aspects of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples or figures represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute a mode for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific examples disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. Example 1

(Preparation of [(ºSiO-)_xTiH_y] [SiH_z] )

[0050] [(ºSiO-)Ti(-CH₂CMe3)3] was prepared by grafting Ti(-CH₂CMe3)₄ precursor on fumed silica, (Aerosil® Si0₂ (dehydroxylated at 700 °C) using the methodology of Basset el al., ( Organometallics , 2006, 25 (15), 3743-3760) and as described below. Ti(-CH₂CMe3)₄ was prepared by the reaction of Ti(0-CH₂-CH3)₄ with Li-CH₂CMe3 as described below. Preparation of the active complex [(ºSiO-)xTiH_y][SiH_z] was carried out in the presence of H₂ at 100 °C and 150 °C. This generated hydride species was used for the activation of N₂ forming NLb.

[0051] All experiments were carried out by using standard air-free methodology in an argon-filled glovebox, on a Schlenck line, or in a Schlenck-type apparatus interfaced to a high- vacuum line (10 ⁶ Torr).

[0052] Preparation of Neopentyl Lithium (IK L/i( 'Me 3) was performed using the methodology of Schrock, etal ., (./Am Chem. Soc., 1978, 100, 3359-3370.). Neopentyl chloride (about 10 g) and finely chopped Li wire (about 3 g, 1% Na) in hexane (0.1 L) was stirred and refluxed under argon for 1 week. LiCl and excess Li were removed by filtration through a medium porosity glass fritted Buchner funnel and the neopentyllithium isolated from the filtrate by cooling and by reducing the volume in vacuo , yielding of white, crystalline LiCH₂CMe3 (5-6 g, 70- 80 %). The product was sublimed at 150 °C.

[0053] Synthesis of tetraneopentyltitanium, TiNp4 or [Ti(CH2CMe3)4] was prepared using the methodology of Dufaud (Angew. Chem., Int. Ed. 1998, 37:806-10), and Maksimov et al., (J. Mol. Cata.,. 1978, 4: 167-79). The prepared neopentyllithium was added to pentane (50 mL of a 0.65 M solution, 32.5 mmol). To this solution was added Ti(OEt)₄ (1.5 mL, 7.3 mmol) in pentane (20 mL) dropwise over 45 min at -78 °C in a foil-covered flask. After the addition was complete, the reaction mixture was stirred for 1.5 h at -78 °C and then for 6 h at room temperature. The solution was filtered, and the solvent was removed under vacuum to leave an oily brown solid. The brown solid was purified by sublimation (55 °C, 10 ³ Torr) over 10 h to give the product

[0054] Preparation of [(ºSiO-)xTiNp3] by grafting TiNp4 onto SiO2-(700). Aerosil® silica was first dehydroxylated at 700 °C to produce the support (SiO₂-700). A mixture of TiNp₄ (0.234 g, 0.59 mmol, 1.2 equiv.) in pentane (15 ml) and SiO₂-(700) (2.53 g, 0.59 mmol of ºSiOH) was stirred at 25 °C for 2 h in a double Schlenk with a small pore silica frit. After filtration the solid was washed three times with pentane and dried for 15 min under vacuum at 25 °C.

[0055] Preparation of active complex hydrides [(ºSi-0-)_xTiH_y] [SiHf. Anhydrous H₂ (850 mbar) and the surface complex (ºSiO-)Ti-Np3 (0.3 g, 0.056 mmol of metal) were heated at 100-150 °C for 15 h in a glass reactor tube (480 ml), in the dark to give [(ºSiO-)xTi-H_y][SiH_z]. Literature reports indicate a greater extent of hydride formation in the temperature around 150 °C.

[0056] Characterization. Preliminary characterization was performed by NMR and infrared spectroscopic measurements, ICP and CHN elemental analysis.

Results

[0057] NMR analyses. 'H and ¹³C NMR spectra of TiNp₄ or [Ti(-CFl₂CMe3)₄] and [(ºSiO-)xTiNp3] are provided in FIG. 2(A) and (B) and that of grafted complex in FIG. 3(A) and (B). The ¾- NMR spectrum of [Ti(CH₂CMe3)₄] displays mainly two signals at 1.18 and 2.243 ppm corresponding to CH: and CHi of Ti-CH₂-C-(CH3)3 respectively. ¹³C NMR of [Ti(CH₂CMe3)₄] show the corresponding carbons around 34 and 119 ppm respectively. 'H- MAS and ¹³C CP -MAS NMR spectrum of [(ºSiO-)_xTiNp3] prepared after grafting [Ti(CH₂CMe3)₄] on SiO₂-700 showed signals at 1.2 and 2.5 ppm for the proton of neopentyl C Hi and C H and the signal around 34 and 114 ppm for the corresponding carbons (FIG. 3).

[0058] ¹H-MAS NMR spectra of [(ºSi-0-)_xTiH_y] prepared after the hydrogenolysis of [(ºSiO-)xTiNp3] at 100 °C and 150 °C show the proton signals at 1 and 2.2 ppm for the proton of the residual neopentyl CHi and C H on the surface. Other main signals are at 4.5 ppm corresponding to Si-H_z species, 6.5 ppm corresponding to the Ti=CH₂ (carbene) and the signal around 8 and 14 ppm shown in the magnified range corresponding to the various hydride species of titanium on the surface of silica (FIG. 4 and FIG. 5). Most probable species expected are mono or bis hydride. In order to confirm the nature of the hydrides ¾-¾ 2D Double quantum NMR measurements were carried out.

[0059] IR analyses: Infrared spectra were recorded for pellets of diameter ~l cm on a Nicolet 6700 spectrometer using an infrared cell equipped with CaF₂ windows, allowing in situ monitoring under controlled atmospheres. Typically 16 scans were accumulated for each spectrum (resolution, 4 cm ¹). [0060] The IR spectra corresponding to [(ºSiO-)_xTiNp3][SiH_z] is given FIG. 6. The IR spectra showed the CH stretching and bending modes of neopentylidiene group around 3000 and 1400 cm ¹. The IR spectrum of the hydride [(ºSi-0-)_xTiH_y] prepared at 100 °C showed the Ti-Hx stretching frequencies around 1715, 1686, 1659, 1642 and 1618 cm ¹ and Si-H and Si-Fh stretching frequencies are around 2262 cm ¹ and 2195 cm ¹. Among the many types of Ti-Hx formed, some specific hydrides are highly active and it even reacts with the 3-5 ppm of N2 available inside the argon atmosphere of the glovebox at room temperature. Hence the starting spectrum contains the chemisorbed MB peak around 3369 cm ¹. The hydride prepared at 150 °C, showed the intense peaks of the hydrides formed around 1618 cm ¹ and the extent of the activity of the hydrides were also high. This was indicated by the intense peaks of the NH stretching and bending vibrations around 3369 cm ¹ and 1609 cm ¹ respectively. Without wishing to be bound by theory, it is believed that the reactive hydrides monohydride and the hydride, which is present in high intensity, are the bis-hydride as indicated by the 'H NMR studies. By increasing the hydrogenation temperature, more extent of Ti monohydrides are formed from bis-hydride, which are highly reactive at the room temperature, and thus formation of more ammonia at slightly elevated temperatures occurred.

[0061] In-situ IR monitoring of N2 and H2 in an IR Cell: A MH_X sample (about 30 mg) was made into a pellet and was placed in an IR cell maintained under controlled atmospheres. The IR cell was evacuated and the IR spectrum was recorded. A side chamber with an adaptor containing 0.6 bar of dry H2 is fitted with the IR cell. IR cell was first filled with 0.2 bar of N2 followed by its exposure to 0.6 bar of dry H2. Reaction progress was monitored by recording the IR spectra after different temperature treatment at definite interval of time. Heat treatments in N2 and H2 at different temperatures such as 25 °C, 50 °C, 100 °C and 250 °C each for 10 hours were tested and the surface reactions were monitored.

[0062] Activity analysis on different batches of [(ºSiO-)3TiH_x][SiH_z] active sites on S1O2 700 °C surface prepared at 100 °C and 150 °C were tested for the reaction of dinitrogen with hydrogen. Screening experiments were done by in situ IR spectral measurements and dynamic reactions in PID reactors under various temperatures. Surface reactions were monitored by in situ FT-IR spectroscopic analysis as shown in FIG. 9 and FIG. 10.

[0063] Elemental analysis of [(ºSi-0-)_xTiH_y][SiH_z] prepared at 100 °C showed around 0.54 % C; 0.17 % H and 1.29 % Ti whereas the one prepared at 150 °C showed 0.41% C; 0.08 % H and 1.2 % Ti. FIG. 9 and FIG. 10 shows the in situ IR analyses of the reaction of Ti- Hx catalyst prepared at 100 °C with N2 and H2 at various temperatures. The IR cell is filled with 0.2 bar of N2 at room temperature. Immediate coordination of N2 was observed and the corresponding new bands appeared around 2342, 2312 and 2280 cm ¹. After adding H2 and heating at 50 °C, new bands corresponding to physisorbed NH3 appeared. The bands corresponding to chemisorbed and physisorbed ammonia are 3369, 3290 and 3179 cm ¹. Upon further heating in N2 and H2 at 100 °C and 250 °C, Ti-H_x bands diminished and the formation of more ammonia was observed. No splitting of NH3 into Ti(NH2)(=NH) was observed in the case of Ti-Hx even after heating up to 250 °C. Though trace amounts of Si-MU was detected around 1550 cm ¹ which might be formed because of the interaction of gas phase NH3 with siloxane bridges as the reaction was under static condition at higher temperatures. The release of the adsorbed NH3 above 150 -250 °C, suggests that the regeneration of Ti-H_x sites could be possible. Ti-H_x complex is much more active than TaH_x complex. NH3 formed at room temperature is strongly bound to Ti and the release of NH3 happens upon heating to 50 °C or above.

[0064] FIG. 10 shows the IR spectra corresponding to the in situ reaction of titanium hydride prepared at 150 °C with the N2 and H2 at successive heat treatments. Reaction of N2 and H2 in presence of TiH_x at room temperature indicated the formation of ammonia, but the ammonia formed at room temperature was chemisorbed on to the surface. The IR Cell is heated to 150 °C to see whether the extent of ammonia formed is increased. More extent of physisorbed ammonia was detected and the silicon bis-hydrides were all disappeared after the reaction. No splitting of ammonia was observed after prolonged reaction hours at 150 °C, but the physisorbed ammonia was released from the surface.

[0065] Reaction with N2 and H2 in a PID dynamic reactor: About 200 mg of active hydride complex was loaded into a stainless steel reactor of half inch diameter, which can be separated from ambient atmosphere. The reactor was then connected to a PID reaction chamber. Gas lines were first purged using dry N2 to remove any traces of air and moisture. Reactant gases, N2 and H2 in the ratio 1 :3 were then flowed to the catalyst chamber and the total flow was 4.2 ml/min. The outlet was connected to an acid trap maintained at 0 °C containing 10 ⁴ M solution of H2SO4. NH3 formed was trapped in the acid. Quantitative estimation of NH3 being formed was done by volumetric titration of the definite amount of acid withdrawn from the trap over a period of time. Dynamic reactions were done at different temperatures by a temperature ramp while allowing the reaction for 10 hours at specific temperatures such as 25 °C, 50 °C, 100 °C and 250 °C.

[0066] Dynamic reaction activities of [(ºSi-0-)xTiH_y][SiH_z] prepared at 100 °C and 150 °C are provided in Table 1 and Table 2. About 200 mg of catalyst was used for the reaction with an active Ti content of 0.0539 mmols for the former and 0.0493 mmols for the latter catalyst.

Table 1. Dynamic reaction activity analysis of (ºSi-Q-)xTiH_yl SiHzl prepared at 100 °C

1.29 1 :3 250 10 4.4 16.2

Table 2 Dynamic reaction activity analysis of (ºSi-Q-)xTiH_yl SiH_zl prepared at 150 °C

[0067] Catalytic amounts of ammonia were formed in both the cases. Surprisingly the activity of a hydride prepared at 150 °C was almost double as that of the one prepared at 100 °C reaching a TON ~30 while the reaction temperature was ramped to 250 °C for 10 hours. It is believed that formation of more extent of active hydride content on this catalyst occurred. This catalyst showed catalytic amounts of ammonia even at room temperature and atmospheric pressure. With increase in reaction temperature both the amount of ammonia and TON increased as represented in the FIG. 11 and FIG. 12.

Claims

1. A transition metal hydride catalyst comprising a support having

[(ºSi-0-)xMHy][SiH_z] on the surface of the support, where M is a Colum 4 transition metal, x is 1 to 3, y is 1 to 3, and z is 1 to 3.

2. The catalyst of claim 1, wherein M is Ti, Zr or combinations thereof, preferably Ti.

3. The catalyst of claim 1, wherein in the support is a non-porous silicate, mesoporous silicate, a microporous silicate, or zeolite support.

4. The catalyst of claim 1, wherein the support has a specific surface area (BET) in a range of 0.1 to 1200 m²/g.

5. The catalyst of claim 1, wherein the support further comprises of alumina, titania, zirconia, chromia or combinations thereof.

6. A method of producing a transition metal hydride catalyst of any one of claims 1 to 5, comprising:

(a) grafting a Column 4 transition metal precursor to a silica support at a

temperature between -80 °C to 300 °C ; and

(b) activating the grafted precursor by contacting the grafted precursor with H 2 at a temperature of 25 °C to 250 °C preferably, from 100 °C to 200 °C.

7. The method of claim 6, wherein the Column 4 transition metal hydride precursor to silica molar ratio is about 1 : 1 to the amount of anchoring surface groups on the silica.

8. The method of claim 6, wherein the activating temperature is about 150 °C.

9. The method of claim 6, wherein Column 4 transition metal precursor comprises a

Column 4 transition metal with alkyl, alkylidene, or alkylidyne or allyl or benzyl ligands.

10. The method of claim 6, wherein the Column 4 transition metal precursor has a

formula of M(R¹)₄, where M is a Column 4 transition metal and R¹ is a saturated or unsaturated C3 to C10 linear or branched alkyl or alkyl residues.

11. The method of claim 10, wherein the Column 4 transition metal precursor is

M(CH₂CMe3)4 where M is the Column 4 transition metal.

12. The method of claims 6 to 11, where M is Ti, Zr or combinations thereof.

13. The method of claim 6 to 12, wherein the Column 4 transition metal is titanium (Ti).

14. The method of claim 6, further comprising preparing the M(R')₄ precursor by reacting M(OR²)₄ with Li-(R') or Zn (R²)2, where M is the column 4 transition metal, R² is a C2 to C₄ alkyl, and R¹ is saturated or unsaturated C3 to C10 linear or branched alkyl.

15. The method of claims 6 to 14, wherein the support has a specific surface area (BET) in a range of 0.1 to 1200 m²/g.

16. The method of claims 6 to 14, wherein the support is non-porous silicate, mesoporous silicate, microporous silicate, or zeolite support.

17. A transition metal hydride catalyst produced by the method of claim 6, under infrared spectroscopy, exhibits at least one or more absorption bands at 1642 cm ¹ and 1686 cm ¹; under proton nuclear magnetic resonance (solid ¹H-NMR) at 400 MHz, exhibits chemical shift values at 8.5 ppm and 14 ppm corresponding to the hydrides.

18. A method of producing ammonia comprising contacting nitrogen (N2) and hydrogen (H2) with a transition metal hydride catalyst at a temperature of 20 to 350 °C and a pressure of 0.1 to 1.5 MPa producing ammonia (NH3), wherein the transition metal hydride catalyst comprises [(ºSi-0-)xMH_y][SiH_z] on the surface of the support, where M is a Column 4 transition metal; x is 1 to 3; and y is 1 to 3, and z is 1 to 3.

19. The method of claim 18, wherein the Column 4 transition metal hydride catalyst comprises [(ºSi-0-)xMH_y][SiH_z], where x is 1, y is 2, and z is 2.

20. The method of claims 18 to 19, wherein M is Ti.